DE69627364T2 - IMAGING DEVICE - Google Patents

Description

technical area

The invention relates to an ink jet imaging device, in particular such a device that several print heads for multi-color printing contains.

Wallpaper end technology

An inkjet system, one of Inkjet recording systems, is a system in which one Nozzle that with from an ink tank supplied Ink filled is about has a heater that is driven by a pulse signal to heat the nozzle so by the pressure of an air bubble caused by the warming in the ink is generated, an ink droplet is ejected. For an imaging device using such an ink jet recording system an image is created using a printhead that is made up of consists of several nozzles aligned in a line.

As in the 11 is shown, a write head mounted on a carriage 3 (hereinafter referred to as "head") adjusted in a main scanning direction (X) by successively several columns 17 individually on a sheet of paper 15 to print to create a band of an image. Then the paper sheet 15 in a subscanning direction (Y) to produce a second band of the image adjacent to the first band. To create a full color image, multiple printheads are used, the ink droplets of different colors, e.g. B. Cyan C, Magenta M, Yellow Y, and Black K to print so that the colors overlap.

However, a print operation with multiple printheads of different colors as described above for producing a full color image suffers from the following disadvantages. As in the 12 there may be a misalignment or deviation D with respect to the relative position between the heads in the transverse or main scanning direction. Such a deviation D1 leads to a vertical stripe pattern in a printed image. The 12 shows an example in which only the head for magenta M is misaligned to the left by a value D with respect to the other heads. In a similar way it could, as in the 13 there is a deviation D2 in the vertical or sub-scanning direction between several heads. Such a deviation D2 causes a horizontal stripe pattern to appear in a printed image. The 13 shows an example where only the magenta M head is misaligned downward by a value D2 with respect to other heads. For example, the difference between heads can affect a printed image.

There is an ink jet imaging device that uses a linear scale to eject droplets of ink 301 with slits 303 that are present along this regularly for each point position, and a linear sensor 302 that along the linear scale 301 is adjustable to detect the presence / absence of slots at any position thereof, as in the 14 shown, synchronized to eject ink droplets at precise points corresponding to individual positions in the main scanning direction of the heads. In this type of image forming apparatus, when a bidirectional (or branch) printing is performed, printing is done on both the forward and backward paths of the heads moving along the main scanning direction, as in FIG 15 (a) is shown, a delay time D1 is generated in the forward path from the detection of a slot until an ink droplet is actually ejected, while a delay time D2 is generated in a similar manner in the reverse path. So the sum of the delay times has the value (d1 + d2). The sum of the delay times (d1 + d2) can affect a printed image due to the deviations (D5) in the ejection positions of ink droplets between the forward and reverse paths, although attempts have been made to print dots at the same position P. The image impairment is particularly clear when a line drawing is printed. For example, as in the 15 (b) is shown if ideally a vertical line 151 should appear, two parallel, dashed lines 12 printed.

The configuration of a head is divided into two types: an integrated type, in which an ink tank with an associated head is integrated, as in the 16 (b) is shown, and a separate type in which a head 3 separated from an ink tank 3 ' is present as it is in the 16 (a) is shown.

Integrated type printheads are called consumer goods handled, which are exchanged by the user at will, if in the ink tank there is a lack of ink. Therefore, every time a Head is exchanged, its alignment checked, and, if available, a correction should be made.

On the other hand, in a separate type of print heads, when the ink in an ink tank is used up, the user only changes the ink tank so that the write head remains intact at its fixed position. In principle, it is therefore sufficient to correct the above-mentioned deviation from print heads only if products are dispatched from a factory. However, it may be necessary to change a user location header if an error occurs in the header or the like. In this case, a head deviation could occur and it is desirable to do so in the To be able to correct the deviation at the user location.

In order to correct the deviation of heads, it is necessary to accurately grasp the extent of the deviation. The detection of a deviation is carried out as follows. Every time a head is exchanged, a predetermined print pattern or test pattern is recorded on a sheet of paper as shown in the 17 is shown. In this example, a vertically elongated rectangular area a (hereinafter referred to as a reference area) is recorded with a head of a special color (black in this case), which serves as a reference for positional alignment, while successively a black area b, a cyan area c , a magenta area d and a yellow area e (hereinafter referred to as comparison areas) are respectively recorded at instructed positions which are spaced in the transverse direction from the reference area in the above order from top to bottom. These areas a to e are all printed in the same direction (here from left to right). Regarding the areas b to e, some of them for which the heads are deviated cannot be aligned with other areas, although attempts are made to print the areas at aligned positions. The illustrated example shows that the head for cyan has an alignment error that results in a transverse displacement of area c relative to the other areas.

In order to detect pressure deviations when printing out both in the forward and in the backward path of the heads, the region a is printed elongated, as indicated by a dashed line in the 17 is shown. According to this extended part, an additional area f is printed with the head of the same color (black) as that of area a at the same transverse position as areas b to e. Only the area f is printed differently from the other areas in the reverse direction (from right to left). It turns out that due to the above-mentioned delay d1 + d2, the area f is shifted to the left with respect to the area b of the same color.

That in the 17 The printed pattern is shown by a sensor 9 detected, which is attached to the carriage near the head and which optically closes the pattern to calculate the deviation values for each head. In the following, the deviation of the heads is also referred to as misregistration.

As in the 18 is shown, the sensor for detecting the print pattern consists of a light emission element 601 , a light receiving element 602 (e.g. a photodiode) and a lens 603 , The 18 (a) and (B) show a front view and a plan view of the sensor. In the 18 a carriage adjustment direction (main scanning direction) is marked with "X", and a direction perpendicular to the carriage adjustment direction is marked with "Y". That from the light emission element 601 emitted light is projected onto the surface of a sheet of paper, and the reflected light is through the lens 603 through from the light receiving element 602 receive.

If the sensor output signal is small, it will be as in the 19 this sensor output signal is shown by an amplifier circuit 701 converted from a current to a voltage by an inverting amplifier circuit 702 amplified and then in a comparator 703 compared to a predetermined threshold voltage to be converted to digital two-level data and digitally processed.

Such a configuration of an image forming apparatus is disclosed in Japanese Patent Application No. 6-120160 (Patent Laid-Open No. 7-323582) and in US 4,675,696 A disclosed.

However, the printed sheet of paper used to detect misregistration is not necessarily ideally flat, but part or all of the sheet may be raised to a height D0 or may be floating at that height (approximately a few millimeters). When such floating of the sheet has occurred, the illuminating position of the light moves from the light emitting element 601 on the sheet from a position P2 to P1, which increases the distance from the lens 603 to the surface of the printed sheet changes, resulting in a defocused state. For this reason, as it is in the 21 is shown, the sensor output signal So ( 21 (b) ) of the sensor 9 ( 21 (a) ) unstable, and accordingly it is impossible to between the actual printed area 14 ( 21 (a) ) and a variable point 81 on the paper sheet 15 ( 21 (d) ) to distinguish. That is, an accurate two-level digitization cannot be performed with a threshold level Th, causing a pulse in the two-level output signal Bo 86 ( 21 (c) ) is generated, the moving point 81 corresponds, resulting in erroneous deletion of the printed pattern.

Even if the two-level digitization is successful accomplished can be, the amplitude of the sensor output signal varies between moving points and immovable points on the paper sheet, resulting in an error in detecting an edge position of the Two-level output signal leads which affect the accuracy of capturing the printed pattern can.

Furthermore, a user sometimes uses auxiliary paper (e.g., tracing paper) as the recording medium. In this case, as in the 22 is shown from the auxiliary paper 222 less light than from normal paper 221 reflected, so that it becomes impossible to detect the peak value of the sensor output signal So that the printed area 14 corresponds because the insufficient light is below a threshold level Th1. For this reason, the threshold level for two-level digitization should be changed to a lower level Th2 depending on the paper used.

It is an object of the invention to provide an ink jet imaging device that has a printed one Pattern even when a recording medium, on which the pattern is printed, something hovers when the deviation several printheads is detected, or even when the recording medium has low reflectivity.

EPIPHANY THE INVENTION

According to the invention, the following is created: an ink jet imaging device with several attached movable writing heads for Forming an image on a recording medium with a test pattern printing device for printing a specific test pattern on the recording medium by using the multiple printheads; a reading device for reading the test pattern printed by the test pattern printing device by optically scanning the test pattern; an attachment position error detection device, um regarding a reference head, which is one of the multiple write heads, from the positional deviations of the printheads other than determine the reference head; characterized in that the Reading device includes: a light emitting element, the light on projects the recording medium, and first and second Light receiving element, which are separated from each other in a particular Distance (X) are arranged; and the attachment position error detection device includes: a subtraction device for subtracting a first or second light receiving output element from the corresponding other light output element and a device for determining the position deviation Base of the subtraction output.

If with this arrangement, as in the 8th the output signal So2 ( 8 (c) ) of a second light receiving element 22 from an output signal So1 ( 8 (b) ) of a first light receiving element 21 is withdrawn, the movable sections rise 82 and 83 of the output paper corresponding to each other, since changes in the output signals due to floating are small. On the other hand, the output signals corresponding to each area of the printed pattern leave the peaks 84 and 85 of the first and second light receiving elements are intact even if the difference between the two output signals has been formed because the first and second light receiving elements are spaced apart and their output signals change greatly (see FIGS 8 (d) ). Therefore, as in the 8 (e) is shown, the position of the printed pattern is accurately detected even when a movable section 81 is present or the reflectivity of the recording medium is low.

The device is preferably provided with the following: a head scanning device for moving the plurality of writing heads in one Main scanning device across the recording medium and a recording medium conveyor for moving the recording medium in a sub-scanning direction, which is substantially perpendicular to the main scanning device, wherein the first and second light receiving elements are equally spaced from arranged and aligned along a line which are at a predetermined angle relative to the head movement direction (main scanning direction) and the call recording medium conveying direction (Sub-scan direction) runs.

Ie that as it is in the 10 is shown, the first and the second light receiving element 21 and 22 equally far from the light emission element 23 are spaced while the common central axis is through the light receiving elements 21 and 22 is inclined at a predetermined angle (e.g. 45 °) against the head adjustment direction (carriage adjustment direction or main scanning direction) and the recording medium conveying direction (paper running direction or sub-scanning direction).

As in the 10 would be shown if the light receiving elements 21 and 22 would not be tilted (state as shown in the dashed box) when reading the region P3 elongated in the transverse direction, the output signals of the light receiving elements 21 . 22 are generated successively with a time difference with respect to the area P3, which would lead to a change in the subtraction output signal only at the position of the area P3. However, if the vertically elongated area P4 through the light receiving elements 21 . 22 read, as shown by the dashed box, would be the output signals of the light receiving elements 21 . 22 change simultaneously with respect to area P4, which would not result in a change in the subtraction output, even though area P4 is present. To avoid this shortcoming, the light receiving elements are 21 . 22 aligned with a tilt.

Preferably, the mounting position error detection device may include: first and second amplifiers for amplifying the output of the first and second light receiving elements, and a gain adjuster for automatically adjusting the first and / or the second amplifier in such a manner that the output signals of the light receiving elements have the same level when the light emitting element is turned on. This enables an undesirable change in the output level of the light receiving elements due to a shift in the position illuminated by the light-emitting element, as it could occur on the basis of setting errors of the head height or due to various factors during manufacture.

In addition to this arrangement, or separately from it, the mounting position error detection device a first and a second amplifier for amplifying the Output signals of the first and second light receiving element and an automatic offset adjustment device for automatic adjustment of the reference level for the first and / or the second amplifier in such a way that the output signals of the light receiving elements on the same Level while the light emission element is switched off. This enables the difference between the temperature characteristics of the two light receiving elements to handle.

When capturing an area of the Test pattern it is desirable to capture the central position within the width of the area. this makes possible it, the difference between amplitudes of the output signals of the light receiving elements, how they depend of the difference in the light absorption capacity of the respective areas different ink colors occurs to handle.

SHORT DESCRIPTION THE DRAWINGS

1 Fig. 11 is a block diagram showing an embodiment of the ink jet image forming apparatus according to the present invention;

2 Fig. 3 is a perspective view of parts of the embodiment;

3 Fig. 11 is a diagram for explaining a method of processing signals in the embodiment;

4 Fig. 14 is a diagram for explaining a method for detecting printed patterns in the embodiment, with a case (a) for detecting misregistration in the transverse direction and a case (b) for detecting misregistration in the vertical direction;

5 shows a configuration of a sensor with its side view (a) and its top view (b);

6 is a graph showing the relationship between a reflective area on a sheet of paper that reflects light and through that in the 5 shown sensor is monitored, and shows a light receiving area of a light receiving element;

7 Fig. 12 shows the internal configuration of the pattern detection unit in the embodiment;

8th 11 is a diagram for explaining an example regarding the detection of a printed area by the sensor in the embodiment;

9 Fig. 12 shows a change in the position illuminated with light relative to the sensor of the embodiment when a sheet paper floating occurs, showing the associated side view (a) and the top view (b);

10 shows an arrangement of the sensor which is inclined at a predetermined angle;

11 shows an example of a tape as printed by the heads of a known ink jet image forming apparatus;

12 Fig. 14 is a diagram for explaining a printing result in the case that a head is misaligned in the transverse direction relative to other heads in the prior art;

13 Fig. 14 is a diagram for explaining a printing result in the case where a head is vertically misaligned in the prior art;

14 Fig. 12 is a diagram showing the relationship between the heads and a linear scale with slits;

15 Fig. 10 is a diagram for explaining a misalignment in a forward and a reverse path when the heads are used in the prior art for bidirectional or bidirectional printing, (a) showing the doubling of position errors due to the bidirectional printing, and (b) shows noticeable image degradation, especially for line drawing during bi-directional printing;

16 Fig. 14 shows a configuration of separate type (a) and integrated type (b) heads and ink containers;

17 shows a printed pattern for detecting misregistration due to head deviation;

18 Fig. 12 shows a configuration of a sensor for detecting misregistration in the prior art;

19 Fig. 14 shows a configuration of a circuit for processing an output signal of the sensor in the prior art;

20 shows a change in the position illuminated by the light relative to the sensor in the prior art;

21 shows processing waveforms as they are obtained when the output signal of the known sensor is processed;

22 shows output signals of the sensor; as obtained when reading printed patterns on sheets of paper with different reflectivities;

23 shows a diagram for explaining a carriage in which the height of the heads is adjustable;

24 shows an arrangement of the in the 23 shown sensor;

25 shows the relationship between the sensor and associated light spots on a sheet of paper when the head height at that in the 23 shown configuration is changed;

26 is a diagram for explaining the Subtraction output signals of two sensors, as they are obtained when the head height at that in the 23 shown configuration is changed;

27 Fig. 12 shows the internal configuration of a pattern detection unit in a second embodiment of the invention;

28 Fig. 14 is a flowchart showing a processing flow in the second embodiment;

29 Fig. 12 is a diagram for explaining another method of detecting a pattern;

30 Fig. 11 is a waveform diagram showing that the amplitude of a sensor output signal changes depending on respective colors; and

31 is a circuit diagram showing the configuration of a circuit for executing the in the 29 illustrated pattern acquisition shows.

BEST ART TO EXECUTE THE INVENTION

Preferred embodiments of the Invention below with reference to the accompanying drawings described in detail. The above parts are the same Reference symbols are assigned and they are not explained again.

The 1 FIG. 11 is a block diagram showing an embodiment of an ink jet image forming apparatus according to the present invention, and FIG 2 Fig. 12 is a perspective view showing an arrangement of respective parts of the device.

Like it in the 1 and 2 , an ink jet imaging device generally has three parts: an external device 1 with an image scanner, a PC, a CAD device, etc., a print control unit 2 and heads 3 , An ink jet imaging device with such a configuration generally operates as follows. The print control unit 2 performs predetermined processing on image data VDI performed by the external device 1 are fed, and then generate the heads 3 an image on a sheet of printing paper based on the processing result.

More specifically, the print control unit has 2 via a CPU (central processing unit) 4 , Head control units 5 , a pattern detection unit 6 , a coverage error detection unit 7 for detecting the values of deviations of respective heads based on that by the pattern detection unit 6 recorded values, a ROM (read-only memory) 18 by the CPU 4 programs to be executed and pattern data to be printed, and an image memory 19 for the temporary storage of image data. The CPU 4 interfaces with the external device 1 , which supplies the image data VDI, and it controls the overall operation of the print control unit 2 including memories (not shown), I / O devices, etc. When the head control units 5 from the external device 1 received image data VDI received, they perform, under instruction by the CPU 4 , a temporary storage of a few tapes of the image data VDI in the image memory 19 out. The stored image data VDI are subjected to various image processing operations, and the resulting image data VDO become synchronous with the scanning operation of the heads 3 output. The synchronization for the print control of the image data VDO etc. is carried out using a signal LINSCL which is from a linear scale 8th synchronized with the scanning process of the heads 3 is produced.

The head control unit 5 also generates activation signals BENB0-7 for the respective blocks of each head 3 and pulse signals for driving heaters (e.g. signals as required for ejecting ink droplets). In this example everyone has heads 3 over 128 nozzles divided into eight blocks, and accordingly eight block activation signals are used.

The image data VDO, the block activation signals BENB0-7, the heater drive pulse signals HENB, etc., as received from the head control units 5 spent, become heads 3 forwarded where control circuits in the mind 3 Only switch on the heater for those nozzles whose associated image data VDO and activation signals (BENB, HENB) are activated, so that ink droplets are ejected onto a sheet of printing paper to create an image column. Such control is repeated while the heads are 3 move in the main scan direction to form a band of an image. In this case, four heads 3 four head control units are also used in accordance with these heads 5 used. The heads 3 are of the integrated type, with ink containers for cyan, magenta, yellow or black, in order to realize full color printing. In the following description, only one circuit for one of these groups is explained.

There is an open / closed detection sensor on the main body of the device 10 available for the top cover. If the top cover 12 open minds 3 exchanged, and then when the top cover 12 is closed again, a process is started in order to detect coverage errors. Alternatively, this command can be instructed by an operator button (not shown) operated by the user. In operation, a print pattern (test pattern) is automatically printed first, as described in the above 17 is shown. In this embodiment, the width of each area of the print pattern is along the scanning direction of the sensor 9 z. B. a few millimeters. The data of this print pattern are in advance in the ROM 18 saved. Then, after printing out the print pattern, the sensor located adjacent to the heads starts 9 the Reading of the printed pattern to detect misregistration.

Incidentally, in the 2 M1 denotes a motor for locking the carriage in the X direction, and M2 denotes a motor for transporting the sheet of paper 15 ,

In this embodiment, the sensor 9 is attached to the carriage that carries the heads. However, the sensor can 9 be separate from the car.

It will be next on the 3 Referenced to explain in detail the process of registering the coverage error.

First, the sensor scans 9 areas a and b of the pattern so that a difference signal SUB of the output signals of the two light receiving elements by means of a special threshold voltage Th in the pattern detection circuit 6 the pressure control unit 2 is converted into a digital two-level signal Bout. Based on this two-level signal Bout, the distance DST between the two areas in the registration error detection unit 7 receive. The distance DST1 between areas a and b is obtained by counting a reference clock signal CLK during a time from the leading edge of the two-level output signal Bout, as generated by scanning areas a and b, to the following leading edge thereof. At a higher frequency of the reference clock signal, misregistration can be detected with a higher resolution. A similar process is performed on areas a and c to obtain a distance DST2. Furthermore, any distance between respective two areas with respect to areas a and d and areas a and e is similarly obtained. If this data is obtained, it is possible to use the data for areas a and b as reference data, differences ( 0 ) between data to calculate the extent to which a head is misaligned relative to a reference head. The sign (plus or minus) of the difference d0 shows in which direction (to the left or right) the head is shifted in relation to the head of the reference color.

The configuration and operation when capturing a pattern form the characteristic part of the invention, and they will now be described in detail below.

It will be first on the 4 (a) and (B) Referenced to explain the pattern. According to the 4 (a) areas a and b (hereinafter referred to as areas a / b) are printed with reference to one of the heads, and areas c / d / e are printed with other heads. In this example, the black ink tank head is used as a reference. In order to align the other heads, which are provided with ink containers of a different color, with the reference head, the areas a / b with the head with the black ink tank, the area c with the cyan color, the area d with magenta and the area e with yellow ge printed.

In the 4 (a) area b is shown as misaligned to areas c / d / e. This shows a result that the areas in the same reference column should be printed, but the print result caused misalignment printing due to a head shift.

Accordingly, in the 4 (a) shows a pattern for detecting a misregistration in the transverse direction, and in the 4 (b) shows a pattern for detecting a misregistration in the vertical direction.

After such printing patterns have been printed out, regarding the pattern for detecting the cross misregistration, the carriage with the sensor attached thereto becomes 9 moved in a main scanning direction to read the printed pattern. Regarding the pattern for detecting the vertical misregistration, the sensor 9 is moved over the printed pattern, and then a sheet of paper is fed in a sub-scanning direction to read the printed pattern.

In order to detect a printing error in the case of a bidirectional printing, an additional area f can be present, as is shown in FIG 17 is shown.

Next, referring to FIG 5 (a) . (B) and 6 an explanation of the configuration and operation of the sensor 9 ,

The 5 (a) . (B) show the internal configuration of the sensor 9 with a first and a second light receiving element 21 and 22 , a light emission element 23 , a lens 24 etc. As it is in the 5 (b) are shown, the first and the second light receiving element 21 and 22 equally far from the light emission element 23 spaced, and they are arranged adjacent to each other in the carriage adjustment direction X (main scanning direction). In this case, the first and the second light receiving elements consist of a two-part photodiode, but two normal single-chip type photodiodes can alternatively be used.

Also in this case, a lens with a diameter of 5 mm is used, which is arranged so that the image printed on a sheet of paper is twice the size on each of the light receiving elements 21 . 22 is focused. Also, as in the 6 is shown, a light receiving surface (hatched in the figure) of each of the light receiving elements 21 . 22 a size of 1.5 mm × 1.5 mm. The light receiving elements 21 and 22 receive reflected light from respective areas of 0.75 mm × 0.75 mm, with an edge of a center C disposed therebetween. (That is, reflection light is received from an area P1 in an area Q1, while similarly reflected light from an area P2 in an area Q2 Therefore, in this configuration, the two light receiving elements 21 and 22 monitors a total area of 1.5 mm × 0.75 mm (ie area P1 + area P2).

The output signals of the light receiving elements 21 . 22 that have read the pattern on a sheet of paper are processed in the pattern processing unit 6 (see the 1 ) processed in order to recognize sections in which intensity changes occur depending on the pattern.

A detailed configuration of the pattern acquisition unit 6 is in the 7 and their operational waveforms are shown in the 8th shown.

In the 7 identify the numbers 31 and 32 a respective current amplifier circuit, the numbers 33 and 34 identify a respective inverting amplifier circuit, the number 35 indicates a differential amplifier circuit and the number 36 identifies a computer. As already explained, the light receiving elements are 21 and 22 placed at a distance from each other. Therefore, the output signals of the respective light receiving elements vary 21 and 22 who read the pattern on a sheet of paper, with a time difference as in the 8 (b) and (C) is shown. (This time difference depends on the adjustment speed of the sensor 9 ab.) In this example, photodiodes are used as light receiving elements, and those in the 8 (b) and output waveforms shown in (c) represent the output signals of the current amplifier circuits converted from a current to a voltage 31 and 32 the 7 that convert current variations, such as those generated in the photodiodes to light variations, into voltages when the pattern is read.

In addition, as stated above, the output signals of the light receiving elements 21 and 22 of weak level, and accordingly the output signals of the amplifier circuits converted from a current to a voltage 31 and 32 through the inverting amplifier circuits 33 and 34 amplified further, their respective output signals then in the differential amplifier circuit 35 be subtracted from each other.

As in the 8 (d) is shown, the subtraction output signal SUB varies only in those sections in which the printed pattern is present, centered on a reference level (GND). Furthermore, as indicated above, the two light receiving elements receive 21 and 22 the light reflected from an area of 1.5 mm x 0.75 mm on a sheet of paper, the level of levitation of the paper sheet not resulting in any significant change in area (since the area is small). For this reason, even when the paper sheet is floating, the resulting change in the output signal is very slow. Accordingly, when the output signal of the light receiving element 21 from that of the light receiving element 22 is subtracted, the output signals in the floating sections (see the (B) . (C) and (D) ). On the other hand, the peak values of the first and second output signals corresponding to the printed pattern remain in the form of a positive peak value even after the subtraction 84 and a negative peak 85 (see the 8 (d) ). This is because the light receiving elements are spaced from each other in the carriage adjustment direction and because the output signals corresponding to the areas of the printed pattern change abruptly. Accordingly, the portions of the areas of the printed pattern can be accurately grasped even when the sheet of paper is floating.

In addition, some users can use a sheet of low reflectivity paper such as an auxiliary paper. In this case, as stated above, it would be impossible to perform the two-level conversion. Like it from the 22 As can be seen, the sensor output signal for a sheet of paper with a lower reflectivity shows a lower DC voltage level than a sheet of paper with a higher reflectivity, but its change components are essentially retained. This allows the output signal to change only in those sections that correspond to the pattern areas, centered around the reference level (GND) (see the 8 (d) ) when the subtraction between the output signals of the light receiving elements 21 and 22 using the same measures as described above. Accordingly, it is possible to accurately grasp areas of the printed pattern even if the printed pattern is formed on a sheet of low-reflectance paper.

In this way, in a pair of light receiving elements for calculating the difference between their outputs, the subtraction output changes only in those portions corresponding to the areas of the printed pattern, so that a two-level conversion with a fixed threshold level can be carried out, such as described below. The output signal of the differential amplifier circuit 35 is in the comparator 36 is compared with a predetermined threshold level to be converted into two-level digital data, which in turn is in the misregistration detection unit 7 are processed digitally to record coverage errors.

As indicated above, the two light receiving elements are 21 and 22 at the same distance from the light emitting element 23 arranged. As in the 9 (a) and (B) is shown, floating a sheet of paper changes that caused by the light emitting element 23 illuminated position so that the front F of the sheet is closer to the light emitting element 23 is lighter than the back R is. This changes the amount of light that shines on the respective light receiving elements 21 . 22 falls which leads to a significant change in the subtraction output signal. To overcome this problem, the light receiving elements 21 . 22 at the same distance from the light emitting element 23 arranged as already stated. This ensures that when the output signals of the light receiving elements 21 and 22 because of a floating sheet of paper change, they experience an equal change so that the changes in the subtraction output cancel each other out.

Also, as stated above, the first and second light receiving elements 21 and 22 both for reading the pattern for detecting cross-registration errors ( 4 (a) ) and the pattern for detecting vertical misregistration ( 4 (b) ) used. For this purpose, as stated in the 10 is shown, the light receiving elements 21 and 22 attached at 45 ° relative to the main scan axis (carriage adjustment direction x) and the lower scan axis (paper travel direction Y). The reason is as follows: When the sensor (light receiving elements 21 . 22 ) would not be tilted (state as represented by a dashed box) when reading the region P3 elongated in the transverse direction ( 10 ) the output signals of the light receiving elements 21 . 22 successively generated with a time difference with respect to area P3, which would lead to a change in the subtraction output signal only at the position of the area (see FIG 8 (d) ). However, if the vertically elongated area P4 is read with the sensor, as shown by the dashed box in FIG 10 labeled, the output signals of the light receiving element change 21 . 22 with the same timing with respect to area P4, which does not change the subtraction output even though the printed pattern is present. The tilt of the sensor 9 is in place to avoid this deficiency.

It will now be explained how the registered misregistration is used to correct the misprints. First, regarding the correction in the transverse direction, a position for which the discharge of an ink droplet is instructed is corrected according to the error size. For this purpose, the timing of the discharge of the ink droplet is made sooner or later, depending on the sign of the error. Alternatively, you can use the image memory 19 stored data are corrected by a value corresponding to the error. Next, regarding the correction in the vertical direction, a part of the vertically aligned one 128 Nozzles (e.g., 120 nozzles) as mentioned above are used as effective nozzles, and these effective nozzles are selected to be shifted by the value corresponding to the error. However, the method of correcting printing errors is not directly related to the invention and methods other than this can be used.

Through the above configuration and the control process will be the pattern for detecting cross and Vertical misregistration read the deviations from relative positions of heads through a simple control to grasp without affecting by floating a sheet of paper or the type of it, and without complicated control to compensate for the influences.

Next, a second embodiment described the invention.

In an ink jet recording system, a printed paper sheet absorbs ink droplets during printing, which may cause the sheet to curl depending on the density or type of the paper sheet, thereby affecting the portion of the sheet in which printing is performed. To prevent the head scanned over the sheet of paper from rubbing against it due to its corrugation, the cart is 102 on which a head 101 (according to the head 3 in the first embodiment) is attached with a lever 103 to adjust the head height, as in the 23 is shown. On the front surface of the car 102 are step-shaped sliding grooves 232 in place with the lever 103 coupled pins 231 intervention. The pencils 231 are also with blocks 233 coupled. If the lever 103 When moved in the X direction by a user, the paired pins slide within the step-shaped slide grooves to the height of the pins 231 to change. This is followed by a change in the height of the blocks 233 whose undersides with the front rail 103 stay in contact. The car 102 is at its rear part on the rear rail 104 stored, being displaceable in the direction X, and about the axis of the rear rail 104 is pivotable. Therefore move by operating the lever 103 the one on the front rail 106 lying blocks 233 up or down, which causes the cart 103 around the rear rail 104 swiveled, whereby the head is moved up or down in the Z direction. Such a configuration allows the user to adjust the height of the head 101 and accordingly adjust the distance between the head and the paper sheet with several levels (here three levels).

Such a head adjustment mechanism is in PCT application WO 07/3085 / A filed on February 23, 1996 to the applicant.

At the in the 23 configuration shown is the sensor 105 (according to the sensor 9 in the first embodiment) similarly by lifting the head 101 raised because the sensor 105 on the car 102 is attached.

As in the 24 are shown to change the incident light rays on the light receiving elements 202 . 203 depending on a change in the lighting position by the light emission element 201 when a sheet of paper floats to compensate for the light receiving elements 202 and 203 arranged at the same distance from the light emitting element. At the same time is the sensor 105 itself so tilted that the light receiving elements 202 and 203 are oriented at an angle of 40 ° with respect to the main scanning direction (X) and the sub-scanning direction (Y). This is the same as in the first embodiment, which is described with reference to FIG 10 was explained.

However, when configuring the 23 the shape of the on the paper sheet by the light emitting element 201 generated light spot 252 relative to the arrangement of the first and second light receiving elements 202 . 203 tilted. In fact, the luminosity of the light irradiated on a sheet of paper in the spot is not uniform, and accordingly, if a normal spot shape 251 is tilted as it is by the spot shape 252 is characterized, change the amount of light falling on the light receiving elements. As a result, as can be seen in the 26 the subtraction output between the two light receiving elements in the positive or negative direction with respect to the reference level (GND) over the entire paper sheet when the head is raised (SUB2) compared to the normal case (SUB1).

Such an event, such as a deviation of the subtraction result with regard to the output signals of the light receiving elements in a positive or negative direction compared to the reference level, could also be caused by mechanical scattering when the sensor is mounted 109 on the car 102 in the manufacture of products, by an unevenness of luminosity due to the light emitting element 201 , regarding the sensitivity of the light receiving elements 202 . 203 and a scattering of constants of the amplifier circuits for amplifying the output signals of the light receiving elements occur.

With regard to such a problem, the 27 an exemplary configuration of the pattern detection unit 6 shown in this embodiment, in which similar elements have the same reference symbols as in FIG 7 are assigned. In this example there is an amplifier 501 with variable gain factor, an analog-digital (A / D) converter 503 , Digital-to-analog (D / A) converter 504 . 506 new, and the comparator 36 is through comparators 507 and 508 replaced. The amplifier 501 with a variable gain factor is configured to output the signal 1 (203 in this case) of the two light receiving elements 202 . 203 at a command from the CPU 4 amplified with an optional gain factor. When a sheet of paper is fed after replacing a head or at a command from the user to correct misregistration, the light emitting element becomes 201 automatically turns on, and then the gain of the amplifier 501 with a variable gain factor so that it is ensured that the output signals of the light receiving elements 202 . 203 are balanced at the same level. More specifically, the output signal of the differential amplifier 35 via the A / D converter 503 through the CPU 4 monitors, which in turn controls the gain of the amplifier 501 with variable gain factor via the D / A converter 504 so that the output signal remains at the reference level (GND).

In addition, the light receiving elements have 202 . 203 due to a variation in manufacturing over individual temperature characteristics, whereby a difference is generated between their output levels when the ambient temperature changes, resulting in a shift in the output signal of the differential amplifier 35 in relation to the reference level. In order to avoid this in this embodiment, as is shown in FIG 27 is shown, an automatic setting is carried out in such a way that the output signals of the receiving elements 202 . 203 have the same level when the light emitting element 201 is in the OFF state. More specifically, similar to the gain setting for the amplifier 501 with variable gain factor, the output signal of the differential amplifier 35 via the A / D converter 503 through the CPU 4 monitors, which in turn monitors the reference level of an inverting amplifier in the offset matching circuit 34 via the D / A converter 506 established.

With reference to the 28 the operation in this embodiment will be explained.

First, when a command to correct a misregistration after feeding a sheet of paper is issued, the carriage becomes 102 automatically adjusted via the paper sheet ( 281 ), and in the offset adjustment circuit 34 an offset adjustment is carried out in a state where the light emitting element 201 remains off ( 282 ). After the differential output signal is set to the reference level (GND) in the offset setting step, the light emitting element becomes 201 switched on ( 283 ) and it becomes the setting step for the amplifier 501 started with variable gain to bring the differential output signal into line with the reference level ( 284 ). This gain setting changes the gain factor of the amplifier 501 with variable gain and it also changes the offset level when the light emitting element 201 is in the OFF state. To handle this, the light emitting element 201 switched off ( 285 ), the level of the differential output signal is checked ( 286 ), and then the offset setting step is performed again when the level has changed. The above steps are iterated so that the differential output does not change from the reference level even if the light emission element 201 is switched on or off. When this condition is reached, registration errors are corrected and corrected.

According to the with reference to the 28 explained process, it is possible to keep the differentially amplified output signal constant regardless of a change in the head height, a dispersion of different element properties and the mounting position, which makes it possible to implement a two-level conversion without detection errors.

Upon completion of the gain and offset adjustments, the printed patterns for registration errors are read and the two-level conversion is performed in the comparators 507 and 508 executed.

Incidentally, as in the case of the embodiment in which color heads are used for four colors and the entire pattern is read by a set of a light emitting element and light receiving elements, the amplitude of the sensor output signal for each color changes as shown in FIG 10 is shown because a sheet of paper shows a different value of light absorption for each color. The difference between the sensor amplitudes causes a deviation (Dcent) of the central position of the detected pulse width. For this reason, there could be a deviation in the central position if simply the pulse width were detected based on the two-level output signal obtained from the differentially amplified output signal of a comparator in order to obtain the position of the central point.

To overcome this problem, two comparators are also in this embodiment 507 and 508 available, whose reference voltages (Vref1, Vref2) are set to positive or negative with respect to the reference level (GND).

This allows respective two-level conversions for the positive and the negative part of the output signal from the differential amplifier 35 to obtain the width of a printed area based on the respective two-level output signals.

An explanation will now be given for one Procedure of calculating the width of areas of a printed Patterns up to the determination of the error values for the respective areas.

The two two-level signals are in the registration error detection unit 7 is used to obtain the widths of respective areas, and then the width data of each area is obtained by the CPU 4 halved to find the location of the central point of the area.

It will be on the 31 Reference is made to in the example of the internal circuit configuration of the misregistration detection unit 7 is shown in this embodiment. The operation of this circuit is described below with reference to that in the 29 illustrated waveforms explained.

In this circuit, the leading edge of the two-level signal (Bo2), which consists of the positive section of the output signal SUB of the differential amplifier, is first 35 was derived in flip-flops 901 . 902 as well as an AND circuit 901 detected by means of a reference clock signal (CLK), and the trailing edge of the two-level signal (Bo2), which results from the negative section of the output signal SUB of the differential amplifier 35 was derived by flip-flops 904 . 905 as well as an AND circuit 906 detected. Then a JK flip-flop creates 907 a signal (AW) with an activation (effective) period between the two edges. This is a signal that indicates the width of an area. After the AW signal has been generated, a flip-flop 908 and an AND circuit 909 a load signal (LD) for operating an up-down counter 910 generated. At the leading edge of each area, the up-down counter 910 loaded with input data and it performs an up-count during the activation period of the signal PW. In this case, the input signal 8th as an input signal to a selector 918 selected so that a value of 0 (HEX) is entered to start the counting process with 0. When the activation of the PW signal is over, the count of the counter becomes 910 on the output signals of AND circuits 911 . 913 . 914 and a flip-flop 912 read out. Each time the sensor is scanned, a pair is read from a reference area and a comparison area. For this purpose, the AND circuits generate 913 . 914 Strobe signals to ensure that latch circuits 915 . 916 Record width data of the respective areas. The CPU then reads 4 Data from the latch circuits 915 and 916 and halves the data read out to calculate the half-value of the width of the area.

With this. With this arrangement, a width DST (described below) between the central points can always be obtained stably since the position of the central point does not change even if the amplitude of the sensor output signal changes for each color. After calculating the halved values of the area widths, the calculated data from a selector 917 selected. Then the up-down counter 910 and the selector 918 set to a down count (AW / DST is set to low "L") and the same areas are scanned again so that from the carry output (B0) of the up-down counter 910 a carry signal is output at each position of a central point of the two areas. This carry signal is a timing signal CENTDT, which indicates the position of the central point of each area. A flip-flop generates this signal 919 a signal DST indicating the duration between the central points of the areas, in what time a counter 920 the width between the central points counts. After the counting process has been completed, the width data is processed by the CPU 4 read. This data is data D1 between the central points of the areas ab, as in the 29 is shown.

The above process is for the areas ac, the Areas a-d and the areas a – e successively iterates in order to obtain the widths D2, ... for each area pair. After this data is obtained, it is using the data D1 for the areas a – b possible for reference, Differences between data D1 and respective data D2, ... to to calculate to what extent (d0) the Heads in Are misaligned with respect to the reference head. It is also possible to recognize in which direction the head is misaligned, what by Assessment based on the sign (positive or negative) of the Difference occurs.

It is a CPU interface circuit 921 present to the CPU 4 between the selectors 917 . 918 , the up-down counter 910 , the latch circuits 915 . 916 and the counter 920 to join.

As described above, according to the invention the first and the second light receiving element together with the Subtractor for subtracting one of the output signals of the first and second light receiving elements are present from each other, so that the floating portions of a sheet of paper correspond Output signals are canceled, thereby reducing the presence of each Area of the printed pattern to grasp exactly because of the time difference between the output signals corresponds to the printed sample area.

Also are the first and the second Light receiving element equidistant from the light emitting element, while their common central axis at an angle relative to the printhead adjustment direction (Main scanning direction) and to the record carrier adjustment direction (sub-scanning direction) is tilted to thereby cover the print pattern areas for both the main as well as the subscanning direction.

INDUSTRIAL APPLICABILITY

The invention is preferably in an imaging device of the ink type, as applicable with an ink jet, in the separated Heads for several Ink colors are attached to perform full color printing.

Claims (13)

Ink jet image forming apparatus with a plurality of movable print heads attached ( 3 ) to create an image on a recording medium ( 15 ) with a test pattern printing device for printing a specific test pattern on the recording medium ( 15 ) by using the multiple print heads ( 3 ); a reading device ( 9 ) for reading the test pattern printed by the test pattern printing device ( 14 ) by optical scanning of the test pattern; an attachment position error detection device ( 7 ) to determine, with respect to a reference head that is one of the plurality of write heads, the positional deviations of the other write heads than the reference head; characterized in that the reading device ( 9 ) includes: a light emitting element ( 23 ), which projects light onto the recording medium, and a first and a second light receiving element ( 21 . 22 ), which are arranged separately from each other at a certain distance (X); and the mounting position error detection device ( 7 ) comprises: a subtraction device ( 35 ) for subtracting an output of the first or the second light receiving element ( 21 . 22 ) from the output of the corresponding other light receiving element, and a device for determining the positional deviations ( 36 ) based on the subtraction result. The device of claim 1, further comprising a head scanning device ( 8th ) to move the multiple print heads in a main scanning direction (X) across the recording medium ( 15 ) and a recording medium conveying device (M2) for moving the recording medium in a sub-scanning direction (Y) which is substantially perpendicular to the main scanning direction (X), the first and the second light-receiving element ( 21 . 22 ) are arranged at the same distance from the light-emitting element and are aligned along a line which runs at a predetermined angle relative to the head movement direction (main scanning direction (X)) and the recording medium conveying direction (sub-scanning direction (Y)). The apparatus of claim 1, wherein the attachment position error detection device ( 7 ) comprises: a first and a second amplifier ( 31 . 32 ) to amplify the output of the first and second light receiving elements ( 21 . 22 ), and a gain adjuster that automatically adjusts a gain of at least one of the first and second amplifiers so that the outputs of both light receiving elements are at the same level when the light emitting element is in an ON state. The apparatus of claim 3, wherein the mounting position error detection device ( 7 ) comprises an automatic offset adjustment device that automatically has a reference level of at least one the outputs of the first and second amplifiers ( 31 . 32 ) adjusts so that the outputs of both light receiving elements ( 21 . 22 ) are at the same level when the light emitting element is in an OFF state. The device of claim 1, wherein the attachment position error detection device ( 7 ) comprises: first and second amplifiers ( 31 . 32 ) which are the outputs of the first and second light receiving elements ( 21 . 22 ), and an automatic offset adjuster that provides a reference level for at least one of the outputs of the first and second amplifiers ( 31 . 32 ) automatically adjusts so that the outputs of both light receiving elements are at the same level when the light emitting element is turned off. The device of claim 1, wherein the test pattern ( 14 ) comprises: a substantially rectangular reference area which is printed with a first of the plurality of write heads and which extends in a direction substantially perpendicular to the scanning direction of the reading device ( 9 ) and several comparison areas, which have the same shape and are parallel to each other with all write heads ( 3 ) at positions with one in the scanning direction of the reading device ( 9 ) fixed distance from the reference area are printed. The apparatus of claim 6, wherein the attachment position error detection device ( 7 ) comprises a two-level conversion circuit that provides an output of the subtraction device ( 35 ) into a two-level signal, and includes devices that interval from a leading edge (B01) to a following leading edge or from a back edge (B02) to a following back edge of the output of the subtraction device ( 35 ) with the locking device ( 7 ) obtained intervals are compared with respect to corresponding comparison ranges to the positional deviations of the heads ( 3 ). The apparatus of claim 6, wherein the attachment position error detection device ( 7 ) comprises: devices for obtaining center points of the reference area width and the corresponding comparison area widths, and devices for obtaining intervals between the center point of the reference area and that of the corresponding comparison area, wherein the by the determining device ( 7 ) obtained intervals are compared with one another with regard to corresponding comparison ranges in order to determine the positional deviations of the heads. The apparatus of claim 6, wherein the attachment position error detection device ( 7 ) comprises a first and a second two-level conversion circuit, each of which has an output of the subtraction device ( 37 ) convert to a two-level signal, the first two-level conversion circuit performing a two-level conversion with a first threshold level (Th1) to generate positive signals ( 34 ) in the output of the subtraction device ( 35 ) while the second two-level conversion circuit is performing two-level conversion with a second threshold level (Th2) to detect negative signals ( 85 ) in the output of the subtractor, whereby the widths of the areas forming the test pattern based on the outputs of the first and second two-level conversion circuits, the center positions of the obtained widths, and the intervals between the center of the reference area and the centers of the corresponding comparison areas are obtained and the intervals obtained with respect to the corresponding compared areas are compared with one another, so that a positional deviation of the heads is determined. Apparatus according to claim 9, wherein the first and second threshold levels (Th1, Th2) of the first and second two-level conversion circuits are set to positive and negative equidistant levels from a reference representing an output level of the subtractor ( 35 ) at the time when the outputs of the first and second light receiving elements are at the same level. Apparatus according to claim 9 or 10, wherein the fastening position error detection device ( 7 ) generates a signal indicating the width of each test pattern area based on a leading edge (B01) of the output of the first two-level conversion circuit and a rear edge (B02) of the output of the second two-level conversion circuit. Apparatus according to claim 6, wherein the scanning direction of the reading device ( 9 ) is a direction that is the same as either the printhead scanning direction or a direction substantially perpendicular to the printhead scanning direction. The apparatus of claim 6, wherein the reference area and the comparison areas of the test pattern ( 14 ) while multiple printheads ( 13 ) are moved in the same direction and the test pattern ( 14 ) further includes an additional comparison area that is printed while the multiple printheads ( 3 ) in a judge movement in that direction.